feat: add GDPR cookie consent banner and upgrade-to-Pro link

- Cookie consent banner with accept all / essential only buttons
- Uses existing i18n messages (en/fr) and localStorage for persistence
- Animated with Framer Motion, respects dark/light theme
- Free tier users see "Passer Pro →" link next to their badge in sidebar

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/components/ui/cookie-consent.tsx

@@ -0,0 +1,65 @@
+"use client";
+
+import { useState, useEffect } from "react";
+import { useTranslations } from "next-intl";
+import { motion, AnimatePresence } from "framer-motion";
+import { Button } from "@/components/ui/button";
+
+const STORAGE_KEY = "cookie-consent";
+
+export function CookieConsent() {
+  const t = useTranslations("cookieConsent");
+  const [visible, setVisible] = useState(false);
+
+  useEffect(() => {
+    setVisible(!localStorage.getItem(STORAGE_KEY));
+  }, []);
+
+  function acceptAll() {
+    localStorage.setItem(STORAGE_KEY, "all");
+    setVisible(false);
+  }
+
+  function essentialOnly() {
+    localStorage.setItem(STORAGE_KEY, "essential");
+    setVisible(false);
+  }
+
+  return (
+    <AnimatePresence>
+      {visible && (
+        <motion.div
+          initial={{ y: 100, opacity: 0 }}
+          animate={{ y: 0, opacity: 1 }}
+          exit={{ y: 100, opacity: 0 }}
+          transition={{ type: "spring", damping: 25, stiffness: 300 }}
+          className="fixed bottom-0 inset-x-0 z-50 p-4"
+        >
+          <div className="mx-auto max-w-3xl rounded-t-2xl border border-border bg-background/95 backdrop-blur-md shadow-2xl p-6">
+            <div className="flex items-start gap-3">
+              <span className="text-2xl shrink-0" aria-hidden="true">
+                🍪
+              </span>
+              <div className="flex-1 min-w-0">
+                <h3 className="font-semibold text-foreground mb-1">
+                  {t("title")}
+                </h3>
+                <p className="text-sm text-muted-foreground leading-relaxed">
+                  {t("description")}
+                </p>
+                <div className="flex flex-wrap items-center gap-3 mt-4">
+                  <Button variant="outline" size="sm" onClick={essentialOnly}>
+                    {t("essentialOnly")}
+                  </Button>
+                  <Button size="sm" onClick={acceptAll}>
+                    {t("acceptAll")}
+                  </Button>
+                </div>
+              </div>
+            </div>
+          </div>
+        </motion.div>
+      )}
+    </AnimatePresence>
+  );
+}